The present invention relates to the manufacture and repair of gas turbine engine airfoils and, more particularly, to an improved method for depositing material on the tip of a gas turbine engine airfoil using linear translational laser welding to prevent airfoil tip recession during engine operation.
Gas turbine engine airfoils such as blades, blisks and the like are manufactured from materials designed to prevent creep and to be rupture resistant in high stress areas of the airfoil, such as the airfoil root, platform section and lower airfoil sections. The airfoil material is typically not necessarily compatible with the environmental conditions or wear properties necessary to prevent recession or shortening of the airfoil tip caused by oxidation, corrosion or contact between the airfoil tip and the stationary shroud of a gas turbine engine during engine operation. The main portion of the airfoil may also be covered with a thermal barrier coating to protect the airfoil from the operating environment while the airfoil tip is typically exposed and can be readily attacked by exhaust gases, salt, air and other corrosive elements in the fuel and air during engine operation.
To avoid tip recession caused by the operating environment, a material more resistant to the operating environment relative to the parent or base material of the airfoil is deposited, typically by laser welding, on the tip of the airfoil. Without such a tip, rapid recession of the airfoil would occur causing reduced engine performance and increased fuel consumption.
The modern, high heat and high stress resistant materials used to make gas turbine engine airfoils can be extremely difficult t weld. Additionally, the complex geometry of the contour of the airfoils can also cause difficulties in depositing the airfoil tip material by laser welding. Cracking can occur in the parent material of the airfoil during welding or during post-weld heat treatment because of the structure of the airfoil and its heat sink characteristics. The airfoils can have hollow interiors with passages formed therein to permit cooling air to be forced through these interior passages of the airfoil and out cooling holes in the surface of the airfoil to create cooling airstreams across the exterior surface of the airfoil during engine operation. Therefore, the heat sink characteristics of the airfoil will vary across the tip of the airfoil. The concentration of heat form the laser welding apparatus can cause damage to the airfoil in areas of the airfoil with poor heat sink characteristics. Residual stresses caused during the cooling of the deposited weld material can also cause cracking in the weld material and the airfoil base or parent material. Other types of cracking that can occur with laser welding include: cracking in the weld material which is typically associated with the cooling rate of the weld material; cracking in the airfoil base material which is typically associated with the grain boundaries of the base material; cracking in the fusion zone where the weld material and airfoil base material interface which can be caused by the cooling rate of the weld material and also the grain boundaries of the base material and also the interaction between the weld material and base material, particularly if the two are different materials; and cracking during post-heat treatment caused by strain aging of the materials or by restrained thermal expansion.
One method for depositing material on the tip 10 of an airfoil 12 is illustrated in FIG. 1A. A laser beam is focused on tip 10 from a laser welding apparatus 14, such as that described in U.S. Pat. No. 4,724,299 issued to Hammeke. The laser beam is focused through a nozzle 16 and the powdered material to be deposited is also forced by a carrier gas through nozzle 16 coaxially with the laser beam and the heat of the laser beam causes the powdered material to melt and fuse with the airfoil material on the tip 10. The periphery or contour of airfoil tip 10 is traced with laser welding apparatus 14 and weld material is thereby deposited on the periphery of the tip as represented by arrows 18. The deposited weld material may be a high heat, high stress resistant superalloy such as Inconel 738 or the like and airfoil 12 may be made of another high stress, high heat resistant metal alloy such as Rene 125 or the like. This method permits deposition of the weld material in a manner that closely follows that of the airfoil contour or provides build-up of the airfoil tip 10 which corresponds substantially to the near-net shape of the airfoil 12. This method, however, causes residual tensile stresses with force vectors as illustrated by arrows 20 in FIG. 1B which can cause massive weld material and base material cracking during deposition the weld material and also during post-weld heat treatment of airfoil 12.
Another method for depositing weld material on the tip end of an airfoil 12 is illustrated in FIG. 2A. This method can be used on those airfoils which are designed without cooling holes formed in the airfoil tip surface and where the weld material can be deposited across the entire surface area of tip 10. In this method, the principal weld direction substantially follows the airfoil contour at tip 10 as illustrated by arrows 22. The weld material is then deposited by laser welding apparatus 14 in beads 24 of weld material and following a stitch pattern which laterally transverses back and forth across airfoil tip 10 substantially perpendicular to the principal weld direction 22. Weld beads 24 are thus continuously advanced in this zig-zag fashion in the direction of principal weld direction 22. While FIGS. 2A and 2B show principal weld direction 22 as being from a trailing edge 26 of airfoil 12 to a leading edge 28 of airfoil 12, the principal weld direction 22 may be in the opposite direction depending upon the heat sink characteristics of airfoil 12 as determined by the hollow interior airfoil sections or baffles (not shown in FIG. 2A) through which cooling air flows during engine operation and out cooling holes 30 to form cooling airstreams, as indicated by arrows 32 in FIG. 2A, which flow over the exterior surface of airfoil 10 to provide exterior cooling during engine operation.
Because of the complex contour shape of some airfoils, depositing tip material in a principal weld direction that follows the contour of the airfoil can cause uneven heating by the laser welding apparatus 14 and nonsymmetrical deposition of the tip weld material. The deposited tip weld material, therefore, will not always substantially follow the contour or neat-net shape of the airfoil as closely as desired and additional processing steps, such as grinding or deposition of additional material may be required to provide a layer of tip material which has uniform height and that substantially follows the contour of the airfoil. An additional disadvantage of this method is that excess material can build up on the airfoil in area 34 (FIG. 2B) where the weld beads 24 of the stitch pattern are closer together because of the curvature of the airfoil and voids or spaces can exist between the weld beads 24 of the stitch pattern in area 36 on the convex surface of airfoil 12. This requires trial and error adjustment of the stitch pattern to eliminate voids or deposition of excess weld material to provide a uniform layer of tip material.
Another disadvantage of the method illustrated in FIGS. 2A and 2B is that, depending upon the airfoil geometry and the heat sink characteristics of the airfoil, the propagation of the heat front caused by the laser beam from laser welding apparatus 14 may not be predictable as the laser welding apparatus or airfoil is moved back and forth along the stitch pattern to deposit the weld material. The surface of airfoil 12 at point 38 may be relatively thin because of the channel or plenum (not shown in FIG. 2A) within the interior of airfoil 10 from which cooling air flows through cooling holes 30 to form airstreams 32. Heat from the laser beam could build up in a concentrated area, such as at point 38 on leading edge 28 of tip 10, causing melt-down of airfoil 12 at point 38.
A further disadvantage, of the method of following a curved principal weld direction 22 which substantially follows the contour of the airfoil, is depositing tip material on airfoils with a leading edge 28' which curve outwardly from a root 40 toward tip 10 as illustrated by the broken line in FIG. 2A. It is difficult to deposit successive layers of material which each follow the near-net shape of leading edge 28' of airfoil 10. The deposited weld material may simply just fall from the overhanging section of the airfoil, drip down leading edge 28' or form a bulbous mass at point 38 which does not continuously follow the contour of leading edge 38'.